Do you think your school would let you do an additional maths qualification with your GCSEs - might depend on teachers and how much time they have but it would help you hit the ground running with A-level and also stretch you a bit more if you're finding GCSE too easy. Worth asking at least!
Also have a look at the UKMT maths challenge, not sure if that's something you can only do through schools or just sign up to on your own but I did it at secondary school and sixth form and it was pretty fun, I didn't take it super seriously (or do particularly well lol) but I think it is a nice way to challenge yourself - the maths itself isn't that difficult but applying it in the weird scenarios the questions set up really is! Another thing I remember friends of mine who were really into maths doing is the Mathsbombe - I think it's team based and probably aimed more at year 12/13, but I think they release the problems for anyone to look at so you could try having a go this year (it starts in late January) to see how you find it.
Slight disclaimer that I did Psychology at Oxford and kind of hated A-level maths but a few of my friends were applying for Maths and related subjects at Oxbridge and other similar universities and this is what I remember them doing! I don't know if any of this is the kind of thing that looks good on an application/personal statement or whatever but you don't really need to be worrying about that in year 10, these just might be some more interesting ways to keep doing maths and challenge yourself while you do GCSEs.